A crossbar memory array is one which includes a first set of conductive lines which perpendicularly intersect a second set of conductive lines. A programmable memory cell configured to store digital data is placed at each intersection of those lines. One type of device which can be used as a memory cell is a resistive switch such as a memristive device. Using memristive devices within a crossbar array brings up several design constraints. For example, when applying read/write voltages to a selected memory cell, care must be taken to prevent excessive leakage current from passing through unselected memory cells. Additionally, during read operations, leakage current through unselected cells can affect the measurements taken by a sense amplifier.
One way to reduce the leakage currents is to use a memory cell that includes multiple memristive devices which are placed in a complementary manner to each other. Two memristors that are connected in a complementary manner means that when one memristor is in a high resistive (OFF) state, then the other memristor is in a low resistive (ON) state. The combined resistance will always be in a high resistive state. This reduces the leakage current within the memory array.
Two memristive devices are typically placed in a complementary manner to each other by connecting two memristive devices in series. Thus, the two memristors share a center conductor. However, when manufacturing an integrated memristive device, the layers which form the memristive portion of the device are subject to variation. Thus, no two layers of memristive regions are exactly the same. Thus, the two memristive devices placed in such a complementary manner will have different switching characteristics. This may cause problems when building a large array of such memristive devices.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.